Rats are more relevant to humans than mice, both physiologically and pharmacologically. The central HYPOTHESIS behind the proposed research is that embryonic stem (ES) cell- based technologies developed for the mouse can be applied to create gene targeted rat models with rat ES cells. Robust and germline competent rat ES cells have been established (Cell 135:1299-310, 2008. Cell 153:1287-98). The overall GOAL of this project is to develop methods for the efficient and routine production of transgenic and gene knock- out rat models for the study of human disease. To achieve this goal, we will: (1) Develop methods to maximize germline transmission efficiency of rat ES cells. (2) Optimize conditions for robust propagation of germline competent rat ES cells. (3) Produce gene targeted rat ES cells and rats.